If your site has multiple languages and translated entities, each result row in a view will contain one translation of each involved entity (a view can involve multiple entities if it uses relationships). You can use a filter to restrict your view to one language: without filtering, if an entity has three translations it will add three rows to the results; if you filter by language, at most one result will appear (it could be zero if that particular entity does not have a translation matching your language filter choice). If a view uses relationships, each entity in the relationship needs to be filtered separately. You can filter a view to a fixed language choice, such as English or Spanish, or to the language selected by the page the view is displayed on (the language that is selected for the page by the language detection settings either for Content or User interface).